This invention relates to a compensator which adjusts the output of a synchro system in accordance with sensed conditions. Typically, the invention may be used to provide an optimum indication of EPR (engine pressure ratio) at different ambient pressures and temperatures so that optimum EPR will always be indicated at the same nominal value.
Synchros are electromechanical devices. In physical appearance, they resemble an electric motor. They normally rotate at low enough speed so that they can be treated as transformers. Electrically they are, in effect, transformers whose primary-to-secondary magnetic couplings may be varied by physically changing the relative orientation of the two windings. By their inherent physical properties and their mechanical and electrical design, synchros make possible the accurate transmission and reproduction, at some remote location, of any data or information that can be converted to angular rotation.
When a simple synchro system, consisting of a transmitter and receiver, is supplied with rates voltage and its components are electrically connected together, the receiver shaft will accurately and instantaneously indicate or follow any position or movement assumed by the shaft of the transmitter. Sundry systems can be built using torque, control, and resolver synchro units, making the synchro an indispensable link in the control and operation of complex equipment in military and commercial applications.
Typically, a synchro transmitter consists of a single-phase field magnetically coupled to a three phase Y-connected armature. When an AC voltage is applied to the field, a voltage is induced in each of the armature phases. These induced voltages vary directly with the angle between the actual field position (with respect to the armature) and a zero reference position. When the three armature leads of the transmitter are connected to a Y-connected armature of a second synchro, the voltages produce a resultant magnetic field in the second synchro, having the same angular orientation with respect to its zero reference as the transmitter field. When the second synchro has a single-phase field winding connected to the same power supply that energizes the transmitter field, it is called a synchro receiver and the field aligns itself to the same angle with respect to its armature as the transmitter field.
A synchro can be constructed as a differential transmitter if the single-phase field is replaced with a three-phase field. The output voltages of the three-phase field can then be applied to a receiver synchro. The angular position of the armature and field of the differential transmitter modifies the angular information supplied by the transmitter and the resulting voltages applied to the receiver may represent the sum or difference of the two angles supplied, depending upon the relative direction of shaft location.
The EPR of a gas turbine engine is the pressure ratio between the final compressor stage of the engine and the first exhaust turbine stage. This ratio provides an indication of the performance of the turbine at any given RPM to the extent that optimum efficiency can be obtained by adjusting the turbine's EPR.
A critical factor in the operating costs of commercial aircraft and in the cruise range of military aircraft is the fuel economy obtained by the aircraft. A savings resulting from an increased fuel efficiency of a fraction of a percent can result in a considerable annual savings in the cost of running a commercial aircraft and can increase the target range and air time of military aircraft, thereby making it important to provide a pilot or other controller with an indication of optimum control settings for the turbine.
The optimum EPR of individual turbines is known to vary as a result of such things as blade dimensions peculiar to the individual turbine, as well as other manufacturing tolerances. The optimum EPR can be determined by using test bench facilities. Therefore, on a four-engine aircraft, each engine may have a different optimum EPR, requiring four different EPR settings. In order to avoid re-calibrating cockpit instruments every time a turbine is removed and replaced on a commercial aircraft, an additional component called a resolver (shown in FIGS. 1-2) is used. The resolver uses a differential transmitter which is electrically connected between a transmitter servo associated with an EPR sensor and a receiving servo associated with a cockpit display of the EPR. The resolver component should not be confused with the use of the term, "resolver" to refer to a synchro with a two-phase signal line, as typically the resolver component described in this application utilizes a three-phase differential transmitter.
The cockpit display typically has an indication thereon concerning an optimum EPR value. Since the turbines are normally tested in a test cell prior to delivery, it is possible to make an emphirical determination, prior to delivery, as to the optimum EPR for the individual turbine. The resolver component is adjusted by rotating one set of Y-connected windings with respect to another set by an amount which will cause a desired phase shift in the resolver component's output from its input. The different electrical phase angle results from the resolver component's coil windings being physically in an out-of-phase relationship with each other. The resolver component is conveniently mounted on the side of the turbine as an accessory, an arrangement which automatically results in the proper EPR gauge being affected.
The optimum EPR for the turbines is known to change in accordance with changes in the ambient temperature and pressure in which the turbine is operating. For commercial aircraft, the optimum EPR is therefore selected from an estimated altitude, such as 9,000 meters. The pilot may also be given charts which indicate optimum EPRs in accordance with different flight levels. Because at altitudes in excess of approximately 10,000 meters, atmospheric temperature ceases to drop, a reading of pressure is insufficient to provide an accurate indication of air density and consequently is insufficient to provide a direct relationship to optimum EPR. For this reason, optimum ratios would vary, not only with ambient pressure, but also with ambient temperature, which does not exhibit a linear relationship with pressure.
The resolver components currently used to compensate for individual differences in optimum EPRs actually add a "fudge factor" to a pilot's EPR reading by providing the pilot with a reading which is altered so that the optimum EPR corresponds to the estimated optimum EPR indicated on the pilot's indicator. For example, the optimum EPR for a given model of turbine may nominally be 12.2. If an individual turbine of that model is predicted during test procedures to have an optimum value of 12.3, the resolver component will be calibrated to provide the pilot with a reading of 12.2 when the measured EPR is actually 12.3. This reading would provide an optimum indication which is calibrated for each turbine but would not provide an optimum indication for all ambient temperature and pressure conditions. While at most flight levels, these optimum EPR's can be predicted in accordance with temperature and pressure, the EPR reading is not altered in the prior art to compensate for these changes.
Accordingly, it is an object of this invention to provide apparatus which adjusts a differential in accordance with sensed conditions in order to provide a modified output of a synchro in accordance with the sensed conditions. More specifically, it is an object of the invention to provide a means to compensate for pressure and temperature conditions and to provide an optimumm EPR reading for an aircraft.